The objective of this proposal is to determine the role of phosphorylation of the AChR in the localization and stabilization of the AChR at the synapse. Monoclonal antibodies to upsilon 1, the Mr43,phi-phi-phi protein kinase from Torpedo which we believe phosphorylates the AChR, will be produced and those cross-reacting with the mammalian neuromuscular junction will be further selected. These monoclonal antibodies will be used to isolate the mammalian upsilon 1 on an affinity column and to characterize its physical and biochemical properties. In addition, these monoclonal antibodies will be used to study the appearance of mammalian upsilon 1 during development in the rat neuromuscular junction and in muscle cells in culture. Finally, monoclonal antibodies to upsilon 1 will be microinjected into living muscle cells in culture to determine whether this protein kinase is involved in receptor clustering.